This is a minimal example for the Raspberry Pi Pico adapted from the TinyUSB examples at https://github.com/hathach/tinyusb/tree/master/examples/host/cdc_msc_hid
I found the examples a bit confusing, because they're designed to support multiple micro-controllers and seem to presume a lot of knowledge so I stripped out everything I could from the configuration and code and checked that it could accept keyboard and mouse input.
To connect a USB keyboard to the Pico you need to connect a Micro USB OTG adapter to convert the Micro USB socket on the Pico into a full size USB socket. It's the same cable that the Pi Zero uses for keyboard input, e.g. https://uk.pi-supply.com/products/pi-zero-usb-adaptor-usb-otg-host-cable
There's only one USB socket on the Pico, so if you use it as a USB input, then you can't use the USB as a serial output for reading the console, or to power the Pi.
To power the Pi, you can apply power to pin 40 - see pinout at https://www.raspberrypi.org/documentation/rp2040/getting-started/#board-specifications
To read the serial output, instead of using the built-in USB, you can use a USB serial dongle connected to pins 1, 2 and 3 on the Pico. I used https://www.amazon.co.uk/DSD-TECH-adapter-FT232RL-Compatible/dp/B07BBPX8B8/ref=pd_lpo_1?pd_rd_i=B07BBPX8B8&psc=1
With that in place, I could run minicom on my computer and see the mouse and keyboard reports from the Pico.
The main.c file is the entrypoint of the program. It starts up TinyUSB with board_init() and tusb_init(), then runs the TinyUSB tasks with tuh_task() and the blink_led_task() to blink the LED to show that the program is running.
The code related to handling USB input is in hid_app.c, this file is included as a source file in the project within the CMakeLists.txt file.